1. Field of the Invention
The present invention relates to a light beam scanning apparatus that scans a light beam based on image data, over a photosensitive drum. The present invention also relates to an image forming apparatus to which the light beam scanning apparatus is applied.
2. Description of the Related Art
In a conventional image forming apparatus, a laser driving circuit causes a laser to emit a light beam by supplying a specified DC current (bias current) to a laser and in addition to this current supply, supplying a switch current that is switched in accordance with image data. If the laser is supplied only with a current that does not exceed a threshold current, the laser does not emit a light beam. The laser emits only faint diffuse light with a varying phase. The quantity of light in the faint diffuse light is much smaller than that in the light beam emitted by the laser supplied with a current exceeding the threshold current. To allow the laser to emit a light beam, it is necessary to supply a current exceeding a predetermined criterion for the internal energy of the laser. Thus, the emission of a light beam can be controlled by supplying the laser with a current not exceeding the threshold current and then slightly increasing or reducing the current compared with the supplied current.
The time from the start of supply of a switch current to the laser already supplied with a bias current until the switch current exceeds the predetermined criterion for the internal energy of the laser is shorter than the time from the start of supply of a required current to the laser supplied with no bias current until the switch current exceeds the predetermined criterion for the internal energy of the laser. Thus, pre-supplying the laser with a bias current improves the emission characteristics of a light beam from the laser. This is why conventional laser driving circuits apply a bias current to the laser.
As described above, the laser element is characterized in that the quantity of light emitted by the element varies in proportion to the supplied current. Accordingly, by controlling the current supplied to the laser, it is possible to control the quantity of light emitted by the laser to form an image. Auto power control (APC) is known to maintain a fixed laser power.
APC maintains a fixed laser power by using a photodiode built into the laser to detect the quantity of light emitted by the laser, comparing this detection signal with a reference value that is a target value for laser power, and then increasing or reducing the current supplied to the laser.
APC is performed while the laser is actually emitting light. Thus, APC cannot be performed during the formation of an image (in an image area) with no fixed emission patterns. That is, APC is basically performed outside the image area. One horizontal line of image is formed using a light beam emitted by the laser in association with laser power based on the results of APC performed outside the image area.
Furthermore, the laser has a temperature characteristic called droop. The laser is an element that emits light the quantity of which varies with the current supplied. Strictly speaking, however, the variation in the quantity of light is affected not only by the current supplied but also by the temperature. Accordingly, even when the same current is supplied to the laser, the quantity of light emitted varies depending on the temperature of the laser. Specifically, if APC is performed outside the image area and a specific current is continuously supplied to the laser to cause the laser to emit a light beam to form one horizontal line of image, then strictly speaking, the quantity of light in the light beam varies between the former half and latter half of the horizontal line. This is because when the laser emits light, the temperature of the laser itself increases to reduce the quantity of light from the laser.
If an image forming speed is relatively slow, the laser provides a relatively low power and there is a sufficient amount of time to store charges in a capacitor. Accordingly, the impact of droop is relatively small and no serious problems occur. However, as the image forming speed increases and an exposure system is changed from normal underfield scan to overfield scan, an increasingly high laser power is required. As the laser power becomes higher, there is a smaller amount of time to store charges in the capacitor and the impact of droop becomes more marked. Furthermore, a significant variation in the quantity of light from the laser occurs within one horizontal line. Consequently, the resulting image density may be affected. Specifically, the start of the scanning direction (one horizontal line) in which a photosensitive member is scanned is markedly affected by droop, whereas its end is insignificantly affected by droop.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2002-307751 discloses a technique executed before recording image data to measure the time for which image data requires almost continuous lighting, to compare the measured time with a predetermined reference time, and if the continuous lighting time is longer than the reference time, to perform ACP different from the normal one.
As described above, with conventional APC, the density varies along the scanning direction owing to droop. As a result, image quality may be degraded.